An MRI apparatus is an apparatus that measures an NMR signal (echo signal) generated by an object, especially, the nuclear spins that form human tissue, and images the shapes or functions of the head, abdomen, limbs, and the like in a two-dimensional manner or in a three-dimensional manner. In the imaging, different phase encoding and different frequency encoding are given to NMR signals by the gradient magnetic field. The measured NMR signals are reconstructed as an image by a two-dimensional or three-dimensional Fourier transform.
The measured NMR signals are disposed in k-space (data space). In this case, the scanning trajectory of k-space is largely divided into two scanning trajectories of the Cartesian coordinate system and the non-Cartesian coordinate system according to the gradient magnetic field pattern to be applied. The Cartesian coordinate system k-space is data space defined by two or three coordinate axes perpendicular to each other. On the other hand, the non-Cartesian coordinate system k-space is data space defined by the size and the angle of deviation. Hereinafter, a measurement to acquire the scanning trajectory of the Cartesian coordinate system is referred to as a Cartesian measurement, and a measurement to acquire the scanning trajectory of the non-Cartesian coordinate system is referred to as a non-Cartesian measurement.
In the non-Cartesian measurement, k-space of a predetermined unit (unit k-space) is measured while changing the angle of deviation, and is converted into Cartesian coordinate system k-space by interpolation processing called gridding. In this case, since a scan is repeatedly performed near the center of k-space for each unit k-space, the influence of the motion, such as breathing, is averaged. For this reason, in the non-Cartesian measurement result, there is a feature that artifacts in a specific direction due to the movement of an object are not likely to occur.
For the non-Cartesian measurement, there is a method called a hybrid radial method. In measurement using the hybrid radial method, the shape of each unit k-space is generally a rectangle, and is called a blade (for example, refer to NPL 1). By applying the gradient magnetic field under the conditions in which the long axis direction of the blade is a frequency encoding direction and the short axis direction of the blade is a phase encoding direction, an echo signal is acquired.
In the hybrid radial method, in order to obtain a spatial resolution equivalent to the Cartesian measurement, echo signals of (π/2) times are required. Accordingly, the measurement time is increased. For this reason, there is a technique of applying parallel imaging to each blade in order to shorten the measurement time (for example, refer to PTL 1 and PTL 2). In this case, in the hybrid radial method, the parallel imaging operation is performed for each blade. Accordingly, since the amount of processing is small compared with so-called general parallel imaging to expand folds having arbitrary shapes, the reconstruction time is also reduced.